Fix nginx config to serve static assets correctly

Add /assets/ location block to serve JS/CSS with correct MIME types
instead of falling back to index.html.

frontend/nginx.conf

@@ -30,6 +30,12 @@ server {
         add_header Content-Disposition "attachment; filename=visitorkiosk.apk";
     }
 
+    # Static assets (JS, CSS, images, fonts)
+    location /assets/ {
+        try_files $uri =404;
+        add_header Cache-Control "public, max-age=31536000, immutable";
+    }
+
     # Frontend SPA routing
     location / {
         try_files $uri $uri/ /index.html;